1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to imaging of lithographic printing-plate constructions on- or off-press using digitally controlled laser output.
2. Description of the Related Art
In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to inking. The ink-abhesive fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
To circumvent the cumbersome photographic development, plate-mounting and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers. For example, U.S. Pat. Nos. 5,351,617 and 5,385,092 (the entire disclosures of which are hereby incorporated by reference) describe an ablative recording system that uses low-power laser discharges to remove, in an imagewise pattern, one or more layers of a lithographic printing blank, thereby creating a ready-to-ink printing member without the need for photographic development. In accordance with those systems, laser output is guided from the diode to the printing surface and focused onto that surface (or, desirably, onto the layer most susceptible to laser ablation, which will generally lie beneath the surface layer).
U.S. Pat. Nos. 5,339,737, Re. 35,512, 5,783,364, and 5,807,658, the entire disclosures of which are hereby incorporated by reference, describe a variety of lithographic plate configurations for use with such imaging apparatus. In general, the plate constructions may include a first, topmost layer chosen for its affinity for (or repulsion of) ink or an ink-abhesive fluid. Underlying the first layer is an image layer, which ablates in response to imaging (e.g., infrared, or "IR") radiation. A strong, durable substrate underlies the image layer, and is characterized by an affinity for (or repulsion of) ink or an ink-abhesive fluid opposite to that of the first layer. Ablation of the absorbing second layer by an imaging pulse generally weakens the topmost layer as well. By disrupting its anchorage to an underlying layer, the topmost layer is rendered easily removable in a post-imaging cleaning step. This creates an image spot having an affinity for ink or an ink-abhesive fluid differing from that of the unexposed first layer, the pattern of such spots forming a lithographic plate image.
Depending on the particular printing member and imaging conditions, certain performance limitations may be observed. For example, a silicone-surfaced dry plate may exhibit insufficient retention of ink by the exposed ink-receptive (generally polyester) layer. The source of this behavior, however, is complex; it does not arise merely from stubbornly adherent silicone fragments. Simple mechanical rubbing of the silicone layer, for example, reliably removes from the ink-accepting layer all debris visible even under magnification, and well before damage to the unimaged silicone areas might occur. Nonetheless, such plates still may print with the inferior quality associated with inadequate affinity for ink. And while ink acceptance is substantially improved through cleaning with a solvent, this process can soften the silicone as well as degrade its anchorage to unimaged portions of the plate. Solvents also raise environmental, health and safety concerns.
Study of the imaging process and its effect on certain types of plate constructions, particularly those containing thin-metal ablation layers below silicone top coatings, suggests that the observed printing deficiencies arise from subtle chemical and morphological changes induced by the imaging process. Plates based on thin-metal imaging layers require heating to substantially higher temperatures to undergo ablation than, for example, laser-imageable printing plates having self-oxidizing (e.g., nitrocellulose) ablation layers. Particularly when low-power imaging sources are used, the exposure time necessary for catastrophic heat buildup can be significant, affording opportunity for unwanted thermal reactions. For example, the low-power imaging pulse of a diode laser must persist for a minimum duration (usually 5-15 .mu.sec) in order to heat a metal such as titanium beyond its melting point of 1680.degree. C. Because the titanium layer is in contact with the chemically complex silicone layer, these high temperatures can induce reactions that produce silicone-derived products of thermal degradation. The breakdown products combine both chemically and mechanically, and with the titanium layer volatilized, are free to interact with the underlying ink-receptive film surface. That surface, moreover, is also rendered more vulnerable to interaction with silicone breakdown products as a result of exposure to high temperatures, which can melt and thermally degrade the surface of the film so that it readily accepts silicone breakdown products. The adhesion, implantation, mechanical intermixture, and chemical reaction of these breakdown products with the film interferes with its ability to retain ink.
These effects can be better appreciated through more detailed analysis of the imaging process. The intense and protracted local heating of the metal layer required to achieve the necessary ablation temperatures exerts a variety of physical effects on the surrounding internal plate structures. Before the metal layer undergoes any change, a bubble forms, lifting the silicone layer. This bubble most likely arises from gaseous, homolytic decomposition of the silicone layer at the interior interface with the rapidly heating metal layer.
Subsequently, a hole forms in the metal layer, beginning in the center of the exposed spot and expanding outwardly, as a bead of molten metal, until it reaches the rim of the exposed area. Some time after the imaging pulse terminates, the previously lifted silicone settles back. This delay results from the persistence of heat in the silicone and exposed ink-accepting layers due to the relatively low heat-transport rates that characterize polymeric materials. The underlying film a undergoes considerable thermally induced physical changes. The effect of intense heating is typically to impart a porous, three-dimensional texture to the surface of the ink-receptive film exposed by imaging.
The surface energy of the exposed film is much lower than that of the unmodified material. In the case of polyester, for example, surface energies of approximately 25 dynes/cm are observed following dry cleaning, as compared with about 40 dynes/cm in the unmodified material. The observed change in surface energy likely derives from the presence of silicone byproducts mixing with the thermally altered film surface. These byproducts build up over the heat-textured polyester surface, effectively masking that surface. And because the combinations involve chemical as well as mechanical bonds, simple abrasion cleaning is insufficient to dislodge the low-surface-energy silicone. These effects interfere with the resulting plate's acceptance of ink. Low surface energy renders a compound such as silicone abhesive to ink; accordingly, reduction in the surface energy of an oleophilic material will diminish its affinity for ink.